Carbonylation processes are well known in the art. Of particular commercial significance are processes for the carbonylation of methanol to make acetic acid and processes for the carbonylation of methyl acetate to make acetic anhydride. See Applied Homogeneous Catalyst With Organometallic Compounds, Cornils et al., Ed. (Bench Edition) (Wylie, Weinheim, Federal Republic of Germany 2000), Chapter 2, Parts 2.1.2 and following, pp. 104-137. See, also, U.S. Pat. No. 6,458,996 to Muskett; U.S. Pat. No. 6,642,413 to Thiebaut, as well as U.S. Pat. No. 6,114,576 to Leet et al.; U.S. Pat. No. 4,039,395 to Eby; and U.S. patent application Ser. No. 11/116,771 (Pub. No. US2006/0247466) of Zinoble et al.; Ser. No. 10/708,420 (Publication No. US2005/0197508) of Scates et al. and Ser. No. 10/058,547 (Publication No. US2003/0144548) of Huckman et al.
To make acetic acid, one method of choice involves carbonylating methanol in a homogeneous reaction medium wherein rhodium is utilized as a catalyst. Generally, the reaction medium includes catalyst, water, acetic acid, dissolved carbon monoxide (CO), methanol, methyl acetate (MeAc), hydriodic acid (HI), methyl iodide and optionally one or more promoters and/or stabilizers. Methanol and carbon monoxide are fed to a reactor as feedstocks. A portion of the reaction medium is continuously withdrawn and provided to a flasher where product is flashed off and sent (as vapor) to a purification train. The purification train includes a light ends column which removes “light” or low boiling components as overhead and provides a product stream for further purification. A particularly preferred carbonylation process is taught in U.S. Pat. No. 5,144,068 to Smith et al. In this so called “low water” process, an alcohol such as methanol is reacted with carbon monoxide in a liquid reaction medium containing a rhodium catalyst stabilized with an iodide salt, especially lithium iodide along with methyl iodide and methyl acetate in specified proportions. With a finite concentration of water in the reaction medium, the product is the carboxylic acid instead of, for example, the anhydride. The reaction system of the '068 patent not only provides an acid product of unusually low water content at unexpectedly favorable rates, but also exhibits unexpectedly high catalyst stability. That is, the catalyst is resistant to catalyst precipitation out of the reaction medium.
Another method of choice for carbonylating methanol involves utilizing a homogeneous iridium catalyst in the reactor. There is disclosed, for example, in U.S. Pat. No. 5,883,295, to Sunley et al. a process for the production of acetic acid comprising carbonylating with carbon monoxide methanol and/or a reactive derivative thereof, in the substantial absence of a metal promoter and/or ionic iodide co-promoter in a carbonylation reactor containing a liquid reaction composition with an iridium carbonylation catalyst, methyl iodide co-catalyst, water, acetic acid, and methyl acetate wherein there is maintained in the liquid reaction composition: (a) water at a concentration of less than 5% by weight; (b) methyl iodide in a concentration of greater than 12% by weight and (c) in the carbonylation reactor a total pressure of less than 50 bar. See, also, U.S. Pat. No. 5,877,348 to Ditzel et al. and U.S. Pat. No. 5,877,347 also to Ditzel et al.
Frequent production limitations in the purification section of an acetic acid unit are the light ends column and the dehydrating column. The light ends column receives a hot vapor product stream from the flasher and operates to remove most of the methyl acetate (MeAc) and methyl iodide (MeI) from the stream before the product stream is fed forward for water removal to the dehydration column.
In a typical acetic acid methanol carbonylation process, hot high pressure liquid from the reactor is reduced in pressure across a valve and flashed in a lower pressure flasher vessel. The vapors liberated from this step are fed near the bottom of a light ends (LE) tower. Condensed liquids rich in acetic acid are removed from a liquid sidedraw above the feed and fed forward for further purification, while vapors exiting the tower overhead are condensed and fed to a liquid-liquid decanter (LE OH decanter). Conventionally, the light phase from the LE OH decanter is refluxed to the LE tower and the heavy phase is recycled to the reactor feed. Total reflux of the light phase in the LE tower forces a higher concentration of water into the LE tower product sidedraw because of a partial condensation of the feed. This higher sidedraw water content requires high reflux rates in the drying column and results in higher dehydration tower loading. Recycle of the light phase to the reactor will reduce water content in the sidedraw, but the concentration of acetic acid in the light phase from the LE tower decanter may be 15% or more and gets higher as the light phase is recycled. The restrictive vapor-liquid equilibrium (VLE) between acetic acid and water forces significant quantities of acetic acid into the LE column overhead product. As the light phase is recycled, the light phase reflux rate decreases and the problem gets worse.